If I remember, most of the attributed translations from Ueshiba on the subject were about Heaven, Earth, and Humankind. Now, Humankind can also be Man.

As explained (and I'm not saying I have this understood correctly) to me:

Cosmologically, you have the forces of Heaven which represent one direction of energy, you have the forces of Earth which represent the opposite direction of energy and Man resides between the two. Where Heaven and Earth meet, you have man.

Now, take a step back and focus on in/yo ho of Daito ryu aiki. In/yo is yin/yang. The method of contradictory forces. So, in aiki, we have contradictory forces. up/down, out/in, etc. Always both, never one or the other. For Daito ryu aiki, these forces are spirals.

Overlapping the two, we can see that Heaven and Earth are the two contradictory forces of in/yo in Daito ryu aiki. Man is the person working/controlling/activating these forces.

In quite a lot of "unbendable arm" demonstrations, one is focused on some sort of visual imagery going out the arm. Typically, this is a water hose with water flowing out the arm. However, that is only one direction. In in/yo ho, there should be a contradictory flow of water coming into the arm. Granted, in this example, one is using a fire hose so, both directions are water. However, we are also talking basic structure and not the complex contradictory spirals of in/yo ho.

Going back to Ueshiba, we find he says (translated by John Stevens): The heart of a human being is no different from the soul of heaven and earth. In your practice always keep your mind on the interaction of heaven and earth, fire and water, yin and yang.

Ueshiba would visualize himself standing on Ame-no-ukihashi, The Floating Bridge of Heaven. Picture it thus:

The self circulation is the dynamics of the person being "Man". "Man", remember, is that area between Heaven and Earth. Imagine an infinite, ever-changing, ever-shifting geometric plane that is the boundary between two galaxies. One is flowing/moving in one direction, while the other flows/moves in the opposite direction. Each galaxies movement is a spiral motion. So, the area where they touch becomes a multitude of points with infinitely changing energies. For a simpler notion, look at the yin/yang symbol. The curvy "s" line between yin and yang -- that's "Man".

The Bridge of Heaven is the entire universe, comprised of those galaxies above. Fire and water.

Ueshiba is standing on that Bridge, being a point between Heaven/Earth and fulfilling the cosmic paradigm of Heaven/Earth/Man. He is becoming an avatar of the Universe whereby other outside entities that come into contact with him, become one with him.

For Ueshiba, the center of all this is his dantien/hara/whatever. This is where the spirals cross in the body and are controlled/created/changed/adapted/etc.

Then, on top of that, we have kotodama, but that is outside this discussion.

Anyway, if we look to in/yo ho, we find that Daito ryu aiki has the contradictory spirals in the body which are created/controlled by dantien. So, once you can create contradictory spirals and be able to use them, then you are on the path to having them naturally expressed in the body. No more creating -- you are the contradictory spirals. You become the entire yin/yang symbol as you have contradictory forces expressed by your body.

NOTE: The self circulation could also be related to Ueshiba's views of a circle. Ueshiba said (translated by Chris Li): In this thing called Aiki, first describe (draw) a circle. Drawing a circle is, in other words, opposing powers.

Now, Chris makes mention that: the word that is used for "opposing powers" is another way of referring to the "Hachiriki" or "Eight Powers" in "Ichirei Shikon Sangen Hachiriki" and refers to the eight variations of "in" and "yo" ("yin" and "yang") and their combinations into opposing pairs - an active force, a quiet force, a pulling force, a loosening force, a splitting force, a combining force, a melting force and a congealing force (you can see that each "in" force has a matching "yo" force).

So, Ueshiba may be making even more about in/yo in that he is saying that even "Man" is a complex contadictory force itself, standing between cosmic in/yo. Doesn't that blow your mind away? LOL!